1. Field of the Invention
The present invention relates to an optical element and a solid-state imaging device and particularly to an optical element and a solid-state imaging device which permit a technique of selectively extracting only an electromagnetic component at a specified wavelength to be realized by a chemically stable, low-cost, lower-profile structure.
2. Description of the Related Art
Electronic devices including solid-state imaging devices for imaging by photographing objects, such as a digital still camera, a camcorder, and the like, have recently increased. Currently mainstream solid-state imaging devices include a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor, and the like. Hereinafter, these solid-state imaging devices are collectively named “CCD/CMOS image sensors”. The CCD/CMOS image sensors have sensitivity in a wide range from a visible wavelength to a near-infrared wavelength. However, unlike the human eyes, CCD/CMOS image sensors may not discriminate between color information, for example, discriminate between red light and blue light. Therefore, in a usual CCD/CMOS image sensor, a color filter which transmits an electromagnetic wave only at a specified wavelength, such as red (R), green (G), blue (B), a complementary color (cyan (Cy), magenta (Mg), yellow (Ye), green (G)), or the like is provided on the front surface of each pixel. By using such a usual CCD/CMOS image sensor, intensity information of each color is obtained from the intensity of transmitted light, and a color image is formed by signal processing of the intensity information of each color.
For the color filters used in the usual CCD/CMOS image sensor, organic raw materials such as a pigment, a dye, and the like are often used. However, the bonding energy of molecules including carbon and hydrogen which are constituent components of the color filter is substantially the same as ultraviolet energy. Therefore, when the color filter is irradiated with high-energy light for a long time, carbon bonds and carbon-hydrogen bonds may be broken. Therefore, in outdoor use for a long time in which the color filter is exposed to sunlight including ultraviolet light, use in an environment in which ultraviolet light is particularly strong, or the like (e.g., climbing, ski, play in the sea, or the like), the transmission characteristics of the color filter change. As a result, the characteristic of color reproduction of a taken image may deteriorate (refer to, for example, IEEE Electron Device Letters, Vol. 27, No. 6, June 2006, p 457-459).
Therefore, color filters using inorganic materials or photonic crystals have been put into practical use (refer to, for example, Domestic Re-publication of PCT International Publication for Patent Application Nos. 2006/028128 and 2005/013369). Further, color filters referred to as “wire grids” or “metal optical filters” (in the specification, unified to the name “metal optical filters” hereinafter) have been proposed (refer to, for example, Quasioptical Systems, Paul F. Goldsmith, IEEE Press, ISBN 0-7803-3439-6, J. Opt. Soc. Am. A, P. B. Catrysse & B. A. Wandell, Vol. 20, No. 12, December 2003, p. 2293-230, and Nanotechnology, Seh-Won Ahn et al., Vol. 16, 1874-1877, 2005 (LG)).